Vedas

They used certain scriptures called Vedas to arrest polytheism, and took excessive pains to integrate all these different deities under one hood of a universal religion. They very strictly guarded the Vedic body of knowledge, and the copy of the Vedas available with them was considered to be the final copy.

Any changes to the Vedas were needed to be approved by this elite group of sages. If the reasons were justified, they approved the changes and incorporated them into the Vedas.

Otherwise, they rejected it and kept the Vedas intact without any contamination. Any changes, to the extent possible, were always added as appendages rather than their being incorporated into the Vedas. So we have Brahmanas, Aranyakas, and Upanishads as appendages to the Vedas. As more and more philosophy developed, all of them got incorporated into this body as appendages.